In the past few decades, thermoresponsive polymers have been intensely studied to develop new smart materials such as hydrogels, films, and drug nanocarriers. Elastin-like polypeptides (ELPs), in particular, which are derived from the hydrophobic domain of tropoelastin and comprise many copies of the pentapeptide repeat Val-Pro-Gly-Xaa-Gly (VPGXG; SEQ ID NO: 22), have also been very widely studied owing to their LCST-like behavior. With heating above their inverse transition temperature (Tt), the ELPs collapse into a coacervate phase, enabling their use as building blocks for temperature-sensitive smart biomaterials. Many studies have demonstrated the outstanding versatility of the (VPGXG)n (VPGXG; SEQ ID NO: 22) consensus repeat for modulating inverse transition temperatures and in the formation of a range of drug delivery vehicles that can be targeted to matrices, tissues and cells either via passive or peptide- and stimuli-responsive mechanisms. The inverse temperature transition can also be triggered by cations, such as Ca2+, or other ligands, via functionalization of the ELP with a ligand-binding domain. While these studies illustrate the utility of the ELPs, essentially all of the ELPs employed have been recombinant, comprising generally on the order of fifty and even hundreds of pentapeptide repeats. Short synthetic ELPs (e.g., those with fewer than ten pentapeptides) have not been used widely for the thermoresponsive fabrication of nanoparticles, owing to their high transition temperatures. In addition, while many hydrogels and films have been produced from ELPs combined with domains of other structural proteins such as silk and resilin, there have been no reports of short ELP-based nanostructures equipped with such domains.
Short synthetic collagen-like peptides (CLPs), similarly, have been employed widely in studies aimed at collagen folding and at development of therapeutic matrices and molecules. CLPs have been shown to mimic the triple helix conformation of native collagen, and thus have served as model systems for triple helix structure and the stabilization effect of specific amino acid residues in collagens, as well as to mimic collagen fibril formation. Additionally, recent studies have illustrated that single-stranded CLPs have a strong propensity to bind native collagen via a strand invasion process. The high propensity of CLPs for collagen permits detection of minute quantities of collagen (e.g., 5 ng) with substantial promise for staining collagens in human tissues (e.g., skin; cornea; bone; liver), especially those with high ECM turnover (e.g., prostate tumor xenografts, joints, and articular cartilage). Despite this widespread use, the utilization of CLPs as domains in responsive nanoparticles has been described in only a very limited number of reports.
There remains a need for thermoresponsive bioconjugates capable of controlled delivery to target matrices and tissues, and selecting thermoresponsive conjugates having unanticipated properties to enable such delivery.